The present invention relates to an improved method of treating paper products in order to enhance various properties thereof, and more specifically, including the step of treating the paper with superheated steam.
2. Description of the Prior Art
There are four broad classes of pulps produced today, namely, mechanical, chemimechanical, semichemical, and chemical pulps. Three of these, mechanical, chemical, and semichemical, are of concern to the present invention and are briefly discussed below.
Mechanical pulps such as TMP (thermo-mechanical pulp) and CTMP (chemithermomechanical pulp) are prepared by processes such as refining which convert wood chips into the pulp. Pulp yields are typically 90-95% from dry wood. Virtually all of the wood components, such as lignin, present in the wood, remain in the pulp. The term "ultra-high yield" pulps is sometimes used for these pulps.
Value can be added to mechanical pulps by additional treatments such as bleaching. For example, CTMP subjected to alkaline-peroxide treatment greatly increases the brightness and value of the pulp. BCTMP (bleached chemithermomechanical pulp) is sold on the open market for use in grades such as tissue, toweling, printing and writing papers, and paperboard.
Chemical pulps are prepared in much lower yield as a consequence of the different processing conditions. Kraft pulp is the most important example of a chemical pulp. Chips are soaked for several hours. at elevated temperature and pH in a cooking liquor which dissolves the lignin from the wood chips. These delignified chips are then thoroughly cleaned to provide a pulp consisting of long, conformable fibres.
The Kappa number is commonly used to indicate the degree is delignification of kraft pulps. It can be used with pulps having yields up to about 70%. There is essentially a linear relationship between the Kappa number and Klason lignin (i.e., acid-insoluble lignin). For these pulps, the relationship is (TAPPI Standard T236) Percent Klason lignin=Kappa number .times.0.15.
Pulp yields are typically 40-55% based on dry wood for "low yield" kraft pulps. Bleaching of chemical pulps is also done to increase the brightness and commercial value. Unbleached kraft pulp is used extensively in the manufacture of linerboard, one component of containerboard.
Linerboard is typically made commercially using different kinds of pulps for different plies in the same sheet. The bottom liner is frequently made from bottom liner stock of virgin kraft pulp of about 55-60% yield, while the top liner is made from top liner stock of virgin kraft pulp of about 48-50% yield. The higher quality top liner is used to hide the lower quality basesheet and provide a better printing surface. It constitutes about 20-30% of the total linerboard weight (Smook, G. A., "Handbook for Pulp & Paper Technologists", CPPA/TAPPI, 1989).
Semichemical pulping uses a combination of chemical and mechanical treatment to develop the pulp fibres. Pulp yields vary over the wide range of 55-90% based on dry wood. NSSC (neutral sulfite semichemical) pulp typically has a yield of 75%. It is favoured for the medium, or fluting, in corrugated containers due to its high stiffness.
Recycled pulps are becoming more commonplace. OCC (old corrugated containers) pulp is used commercially to make 100% recycled linerboard. Different pulps are used to make the inner fluting and outer linerboard of a corrugated container, as noted above. OCC for linerboard manufacture can be a mixture of virgin and recycled kraft and semichemical pulps. The composition of OCC and the behavior of paper made from this furnish is further discussed.
Paper is frequently manufactured not just from pulp fibre but from a mixture of pulp fibre and inorganic particles. Such paper grades are generally referred to as "filled" papers. A variety of fillers can be used, but clay is a common example. Adding fillers to paper has a detrimental effect on the strength properties, but can improve the optical properties, of the paper.
Previous work has demonstrated that super-heated steam drying of paper made from pure mechanical pulps, such as TMP and CTMP, significantly improves the dry tensile strength of the paper without substantially increasing sheet density. Paper made from pure chemical pulps such as kraft does not have increased strength after drying in superheated steam (Cui, W.-K., Mujumdar, A. S., and Douglas, W. J. M., "Superheated Steam Drying of Paper: Effects on Physical Strength Properties," in Drying '86 [A. S. Mujumdar, Ed.], Hemisphere, N.Y., pp. 575-579 [1986]; Poirier, N. A., "The Effect of Superheated Steam Drying on the Properties of Paper," Ph.D. thesis, Department of Chemical Engineering, McGill University, 1992; McCall, J. M. and Douglas, W. J. M., "Superheated Steam Drying of Paper from Chemithermomechanical Pulp," Tappi J., 77 [2]:153-161 [1994]).
The quality requirements of a sheet of paper are becoming increasingly stringent. As paper machine speeds increase, the strength of the wet web must also be adequate to avoid web breakage. Once dried, the paper is subjected to many different end uses, depending on the grade of paper. The relative importance of the various surface and mechanical properties of the paper depends on the end use of the paper. For example, with tissue and toweling and even for some printing papers and paperboard, bulk is very important. For linerboard, the compressive strength and air resistance are two key properties. Printing and writing papers must have adequate resistance to penetration of liquids, and in some cases higher bulk is also an important property.
For many grades of paper and paperboard, a high bulk (apparent specific volume) is an important property. This is especially true for grades such as tissue and toweling for which the pulp properties and processing conditions are carefully selected to provide a final dry sheet having acceptably high bulk. A prime criterion in the choice of drying technique for these grades is the achievement of high bulk. The importance of increasing the bulk of tissue is demonstrated, for example, by the patent issued to the Kimberly Clark Corp. (U.S. Pat. No. 4,994,144, Chen et al, February 1991). Tissue and toweling frequently contain bleached chemithermomechanical pulp (BCTMP) and bleached kraft pulp (BKP) in substantial quantities.